Conventionally known as a winch device mounted on a crane for conducting hoisting work (crane work) is an electric winch device driven by an electric motor to raise an object for hoisting work. Then, a known electric winch device is provided with a regeneration function of converting, during the lowering of an object, kinetic energy generated due to the moving-down of the object into electric energy and recovering the same. Patent Literature 1 set forth below discloses one example of an electric winch device provided with such a regeneration function.
The electric winch device disclosed in Patent Literature 1 is provided with a motor as an electric motor, and electricity storage means which stores regenerative power generated in the motor during the lowering. The motor is configured to be driven by at least one of electric power supplied from the electricity storage means and electric power supplied from a commercial power source to operate a crane.
Some movable cranes use an electric winch device capable of conducting a moving-down of an object, in which the object is lowered in a state close to the free-fall. When such an electric winch device is provided with the regeneration function as described above, at the time of the free-fall of the object, electric power is regenerated by an electric motor and the regenerated electric power is consumed through storage of electricity in an electricity storage device, resulting in generating a braking force in the electric motor. The braking force generated in the electric motor brakes rotation of a winch drum in a lowering direction, resulting in braking the free-fall of the object.
However, when a fail such as a short-circuit develops in any part of an electric system leading from the electricity storage device to the electric motor, electric power regenerated by the electric motor is not consumed, so that no braking force is obtained by regeneration. Although an allowable stopping height is set as a height of a lowest position at which the object is required to be stopped by braking operation, at the time of the free-fall of the object, when no braking force is obtained by regeneration as described above, the object might move down to a position at a height lower than the allowable stopping height.